The present invention pertains, in general, to phase-locked loops for very wide range acquisition and in particular to phase-locked loops for communications systems in which data is encoded into a plurality of symbols such that a transition occurs between each symbol and such that two like symbols cannot appear successively and methods therefor.
Phase-locked loops have been used in data communication systems to track a data clock signal transmitted along with a data signal. However, in communications systems wherein the data clock is recovered in the receiver from the data stream, ambiguities in data codes that have been used have limited the range of acquisition of the phase-locked loop.
On the one hand, a digital loop using a frequency/phase detector has the capability of unambiguous locking over a wide frequency range, but cannot tolerate the sort of noisy inputs that are experienced in communication systems operating near threshold. On the other hand, a conventional analog loop is not generally capable of distinguishing a lock on a true frequency from a lock on a subharmonic or a lock on a higher harmonic of the true frequency when the input consists of narrow pulses.
Furthermore, even communications systems using an unambiguous encoding scheme cannot be used to full advantage when coupled to existing phase-locked loops which have a narrow, ambiguous locking range. In order to approach the desired capability, existing loops require a very large signal to noise ratio, such as is the case with a digital loop using a frequency/phase detector, require manually assisted coarse tuning to avoid harmonic or subharmonic locking, or require constraints placed on the data bit structure, such as a fixed bit sequence preamble at the beginning of each digital message.